Solutions prepared by molecular weights are called "molar" solutions, and
as far as I know this is not the way it's done in photography! (Scientific
labs, yes - photographic methods aren't that precise!)
The problem you raise is dissolving a solid in a liquid, and how does one
figure percentages. The ideal way would be to weigh the solid to the amount
of the percentage and then add the liquid for its amount by weight. Though
this may sound technically correct, it is not the way the formularies plan
it! What they mean is you measure out - say - half of the total amount of
the liquid (if the total of the solution is to be 500 ml, for example, you
would measure 250 ml) in a beaker or other vessel, add the "percentage"
amount of the solid, stir to dissolve (higher concentrations won't dissolve
completely at this point) then add the remaining liquid up to the total,
stirring to insure uniformity. So:
to make 500 ml final solution, use:
50 grams to make a 10% solution
75 grams to make a 15% solution
225 grams to make a 45% solution, etc.
(To make a liter of solution, you double the quantities given.)
No problem, right?
Sil Horwitz, FPSA
Technical Editor, PSA Journal
silh@iag.net